Electroless or chemical nickel plating solution containing a hypophosphite salt as a reducing agent has been widely used for the surface treatment of electronic parts, precision parts, and plastic articles. As plating reaction proceeds, the hypophosphite used as a reducing agent is oxidized into a phosphite which accumulates in the plating solution. This plating solution becomes unusable when the concentration of phosphite exceeds a certain limit.
A number of methods have been proposed for the disposal of such spent electroless nickel plating solution containing a hypophosphite as a reducing agent. However, there is available no effective method for treating the spent plating solution in an industrially advantageous manner and recovering valuable substances from the solution for reuse.